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SH Lee

Publications and source records attributed to SH Lee.

At least 19 recordsLinked to original sources

Resonance-mediated chemical reaction: F+HD-->HF+D

Conclusive evidence is presented for the existence of a reactive resonance in the F+HD reaction. In a molecular beam experiment, the resonance appears in the integral cross section as a distinct steplike feature, while in the differential cross section it is manifested as sharply varying forward-backward peaks in the product distribution. A detailed analysis of the quantum dynamics establishes that a reactive resonance localized in the transition-state region is responsible for these remarkable observations. At collision energies below 1 kcal/mol, the reaction proceeds almost exclusively through resonant tunneling with very little contribution from the more conventional direct mechanism.

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Efficient syntheses and ring-opening reactions of trans- and cis-oxazoline-5-carboxylates

[formula: see text] cis- and trans-Oxazoline-5-carboxylates were synthesized efficiently from isopropyl trans-cinnamate utilizing the Sharpless AA reaction. trans-Oxazoline was much more reactive than the cis-isomer toward ring opening reactions. From ab initio molecular calculations, the cis-isomer was predicted to be less reactive than the trans-isomer by 2.7 kcal/mol. Both syn and anti acetylthio esters and anti diamino esters were synthesized from these cis- and trans-oxazoline-5-carboxylates.

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A new G-M counter dead time model

A hybrid G-M counter dead time model was derived by combining the idealized paralyzable and non-paralyzable models. The new model involves two parameters, which are the paralyzable and non-paralyzable dead times. The dead times used in the model are very closely related to the physical dead time of the G-M tube and its resolving time. To check the validity of the model, the decaying source method with 56Mn was used. The corrected counting rates by the new G-M dead time model were compared with the observed counting rates obtained from the measurement and gave very good agreement within 5% up to 7 x 10(4) counts/s for a G-M tube with a dead time of about 300 micros.

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Molecular Dispersion of Chains in the Mixed-Valence Complexes

A way to prepare molecular electronic wires in organic media is the solubilization of one-dimensional mixed-valence complexes with varied metal species through the formation of amphiphilic supramolecular assemblies (see picture). Dissociation and reassembly of the complex was detected as thermochromism in the intervalence (M(II)-->M(IV)) absorption bands.

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Preparation and Growth Mechanism of Uniform Colloidal Copper Oxide by the Controlled Double-Jet Precipitation

Monodispersed crystalline CuO particles of different morphologies were synthesized by the controlled double-jet precipitation (CDJP) technique using NaOH and Cu(NO3)2 as the reactants. The effects of temperature, reaction time, pH, and the concentrations of chemicals on the characteristics of final products were investigated. With lower [NaOH]/[Cu(NO3)2] molar ratios and temperatures, colloidal gerhardtite Cu2(OH)3NO3 precipitated, while tenorite CuO particles of various morphologies were obtained at higher temperatures and [NaOH]/[Cu(NO3)2] ratios. Time resolved electron microscopy indicated that nanosized primary particles formed immediately after mixing the reactants, followed by aggregation into larger uniform particles of submicrometer size.

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The Fluorescence Excitation Spectrum of HCO &Btilde;2A'-&Xtilde;2A', 000 Band

The rotationally resolved laser-induced fluorescence spectrum of the HCO &Btilde;2A'-&Xtilde;2A' transition in the 000 band is observed under thermalized conditions. The formyl radical is produced from acetaldehyde photolyzed at wavelengths 310 or 308 nm. Some spin-splittings of the spectra are resolved. About 988 transitions to Ka' </= 3 and N' </= 24, approximately 94% of the observed lines, are assigned in this work including weak DeltaKa = &plusmn;2 transitions that become allowed because of the effects of axis-switching. Copyright 1997Academic Press

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A Point Mutation of Acetylcholinesterase Associated with Azinphosmethyl Resistance and Reduced Fitness in Colorado Potato Beetle

A serine to glycine point mutation of acetylcholinesterase (AChE, EC 1.1.1.7) was identified in an azinphosmethyl-resistant strain of Colorado potato beetle [Leptinotarsa decemlineata (Say)]. The position of the mutation corresponds to Val 238 of the Torpedo AChE and represents the first amino acid residue to form the alpha-helix, alpha-E'1. The predicted secondary structure of the mutation-containing region of AChE suggested that the transition from the turn to the alpha-helix occurs sooner in the sequence when serine is replaced by glycine. Thus, conformational changes in the AChE due to the alpha-helix deformation were expected to impinge upon both the catalytic and the peripheral binding sites, resulting in the modification of the bindings of organophosphorus insecticides and other ligands to these sites. The mutation appeared to be associated with the fitness of the beetle. The intrinsic rate of increase of the azinphosmethyl-resistant (AZ-R) strain was relatively low when the beetles were reared on the Russet Burbank potato cultivar, but was relatively high when they were reared on the NDA 1725-1 potato cultivar. Because these two potato cultivars contain different amounts of steroidal glycoalkaloids (e.g., alpha-solanine and alpha-chaconine), the different fitness of the AZ-R strain on different potato cultivars may be partially attributed to the increased sensitivity of the azinphosmethyl-resistant form of AChE to the inhibition by alpha-solanine and reduced sensitivity to alpha-chaconine as previously reported.

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Advantages of Total Laparoscopic Hysterectomy

We compared 112 total laparoscopic hysterectomies (TLH) with 72 laparoscopic hysterectomies (LH) performed from January 1, 1995, to September 30, 1995. Patient characteristics (age, weight, parity) and indications for surgery were similar between the groups. All surgeries were performed with electrosurgery or suture ligature, or both. Average uterine weight was slightly lower with TLH (193.1 &plusmn; 96.2 g) versus LH (237.4 &plusmn; 84.5 g). Thirty-three women (29.46%) undergoing TLH had had previous pelvic surgery, versus 12 having LH (16.67%). Operating time was significantly shorter for TLH (117.6 &plusmn; 38.2 min) than LH (134.9 &plusmn; 37.4 min). Less bleeding, as indicated by decreased postoperative hemoglobin, was noted with TLH (1.3 &plusmn; 0.7 g/ml) versus LH (1.7 &plusmn; 1.1 g/ml). Fewer cases of serious complications, such as genitourinary tract damage, and less formation of granulation tissues on the vaginal cuff associated with persistent leukorrhea and postcoital bleeding occurred with TLH, probably because more precise surgery can be done under direct vision. We believe TLH can be performed more safely and quickly than LH by an experienced surgeon. A potential advantage of TLH is less postoperative infection due to less vaginal manipulation. Other advantages are the lengthening of the vagina, less postoperative prolapse of the vagina, and less enterocele development because of more precise anatomic restoration of the pelvic structures under direct visualization. Since detailed pelvic structures can be visualized, excised, and restored, TLH has all the possible benefits of subtotal hysterectomy, if any, due to the maximum preservation of supporting structures (cardinal, uterosacral ligaments) and nerve plexus, thus making subtotal hysterectomy obsolete. Further studies and long-term follow-up are required.

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